Bearing assembly for the working spindle of a machine tool

ABSTRACT

A bearing assembly for the working spindle of a machine tool such as a lathe, is disclosed consisting of angular contact ball bearings in a tandem O arrangement with an inside ball bearing, a middle ball bearing and an outside ball bearing. In order for the effective cantilever length in such bearing arrangement to be as short and well defined as possible and while also remaining approximately constant under load, the spindle has a larger diameter in the area of the outside bearing than in the area of the other bearings.

TECHNICAL FIELD

This invention concerns a bearing assembly of the type used in theworking spindle of a machine tool.

BACKGROUND ART

It is known that the working spindle of a lathe, for example, can bemounted in the area of the hub of the spindle in roller bearings withball-cylinder rollers, in roller bearings with tapered rollers, infriction bearings, in hydrostatic bearings or hydrodynamic bearings.Conventional angular contact ball bearings are supplied by themanufacturer with different levels of prestress. Normally bearings witha low prestress are used. In order to achieve optimum conditions withregard to the prestress required in use of double row cylinder bearingswith a tapered bore, it is also known that the taper on the main spindlecan be designed with a positive slope in order to impart a differentprestress to the rows of cylindrical rollers. However, this requires asuitable corresponding design of the bearing bore.

The reason why certain prestress conditions are desired is the fact thatthe cantilever length of the spindle, i.e., the lever arm which acts ona certain bearing and is the determining factor for the bowing of thespindle, should be as short and defined as possible.

With a conventional working spindle bearing with angular contact ballbearings in tandem O arrangement, this cantilever length cannot bedefined precisely in many cases because it is impossible to assure thatthe outer bearing has the greatest prestress and thus also the greatestrigidity. This cantilever length enters the calculation of rigidity withthe third power and can extend into the middle bearing if the prestressof the individual bearings is not defined with sufficient accuracy, andthis has negative effects on the efficiency and accuracy of the entirebearing assembly. This disadvantage appears especially when theworkpiece to be machined is held only in a chuck without additionalsupport.

In order to have the shortest possible cantilever length, it is alreadyknown (German Patent No. 3,233,914) that the bottom of a chuck to bemounted on the spindle flange can be integrated into this flange andthus the effective cantilever length is shortened. However, the thebearing application cannot be improved by this measure.

SUMMARY OF THE INVENTION

This invention is directed toward solving the problems of the bearingassembly of the type defined above, so the effective cantilever lengthis as short and well defined as possible and remains approximatelyconstant under load

In order to solve the above problems, according to the presentinvention, a bearing assembly is provided for the working spindle of amachine tool such as a lathe, which includes angular ball bearings in atandem O arrangement with an inside ball bearing, a middle ball bearingand an outer ball bearing, wherein the diameter of the spindle in thearea of the outer bearing is larger than that in the area of the innerand middle ball bearings. The spindle preferably has a step in the areaof transition between the middle bearing and outer bearing. The sectionof the spindle having a larger diameter preferably extends beyond thearea of the outer bearing. A spacer is provided between the insidebearing and the middle bearing, which consists of an outside ring and aninside ring which are concentric with each other wherein the inside ringpossesses a width which is smaller than that of the outside ring.

Due to the larger diameter of the working spindle in the area of theouter bearing, this bearing has a greater prestress than the otherbearings. The effect of this is that the cantilever length, which is adetermining factor for the rigidity of the spindle, extends onlyapproximately to the middle of this outer bearing. By selection ofappropriate dimensions, it is possible to assure that this optimumcantilever length will remain constant even under the conventional loadsand thus the operating conditions can be defined precisely.

Since it is necessary with bearing applications of the type definedinitially to achieve a balanced design with regard to prestress, thegoal is to have the outer and inner bearings exposed to approximatelythe same prestress, while the middle bearing should be free ofprestress. This can be achieved by using a spacer between the middlebearing and the inside bearing, where the spacer consists of an outerring and an inner ring concentric to the former and having a widthsmaller than the outer ring. When using such a spacer, the lowprestress, which the middle bearing has before it is installed in amachine, is compensated.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will now be explained with reference to the accompanyingdrawings, wherein:

FIG. 1 shows an axial section through a working spindle of a lathe inthe area of the hub of the spindle; and,

FIG. 2 shows an enlarged detail taken from FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The bearing application according to this invention is an arrangementhaving angular contact ball bearings in a tandem O arrangement with anouter bearing 11 and a middle bearing 12, both of which have lines ofaction inclined by the same angle, and with an inner bearing 13 having aline of action inclined in the opposite direction. Bearings 11 and 13are arranged in the bore of machine housing 17 of a lathe and sit on theworking spindle 16.

As shown in FIG. 2, spindle 16 has a step 21 in the area of outerbearing 11 and thus has a larger diameter than in the area of middlebearing 12 and inner bearing 13. Due to this larger diameter, outerbearing 11 has a greater prestress than the other bearings, so thecantilever length, which is labeled as a in FIG. 1, has a preciselydefined length and extnds only approximately to the middle of thisbearing, whereas if the prestress of outer bearing 11 is not largeenough, i.e., if the prestress of the middle bearing 12, for example, islarger, then cantilever length a can extend as far as middle bearing 12.

The section of spindle 16, which has a larger diameter, extends beyondthe range of outer bearing 11. Outside of outer bearing 11, there is alabyrinth gland 19. In addition, the bearing application is secured atthe outside by a covering ring 20 which is bolted to the machine housing17. The section of spindle 16, which has a larger diameter, is shownwith a thicker line for the purpose of emphasis.

In order to achieve a balanced bearing application with regard toprestress, a spacer consisting of an outer ring 14 and an inner ring 15,which is concentric with the former but has a smaller width than theouter ring, is used between middle bearing 12 and inside bearing 13. Theeffect achieved by using a spacer designed in this way is that the outerbearing 11 and the inside bearing 13 have approximately the sameprestress, while the middle bearing 12 is free of prestress. The bearingapplication is prestressed in the axial direction by a conventionaltwo-step interference fit.

What is claimed is:
 1. In combination, for use with a machine tool:aworking spindle; an assembly of angular ball bearings arranged in tandemrelationship on said spindle and including an inner bearing, a middlebearing and an outer bearing, said spindle having a section of largerdiameter in the area of the outer bearing than in the area of the innerand middle bearings; and spacing means between said inside bearing andsaid middle bearing, said spacing means including an inner ring and anouter ring, said inner ring being concentric with said outer ring andhaving a width less than the width of said outer ring.
 2. Thecombination of claim 1, wherein said spindle includes a step thereindefining a transition between said area of said outer bearing and saidarea of said inner and middle bearings.
 3. The combination of claim 2,wherein said section of said spindle having a larger diameter extendsbeyond said area of said outer bearing.
 4. The combination of claim 1,wherein said section of said spindle having a larger diameter extendsbeyond said area of said outer bearing.